The invention relates to the continuous casting of metals, and more specifically to the continuous casting, directly from liquid metal, of austenitic-type stainless steel strip whose thickness is of the order of a few mm, using the process called xe2x80x9ctwin-roll castingxe2x80x9d.
In recent years considerable progress has been made in the development of processes for casting thin carbon steel or stainless steel strip directly from liquid metal. The process mainly used at the present time is that of casting said liquid metal between two internally cooled rolls, rotating about their horizontal axes in opposite directions and placed opposite one another, the minimum distance between their surfaces being approximately equal to the thickness that it is desired to give the cast strip (for example, a few mm). The casting space containing the liquid steel is defined by the lateral surfaces of the rolls, on which the strip starts to solidify, and by lateral closure plates made of refractory which are applied against the ends of the rolls. The liquid metal starts to solidify on contact with the external surfaces of the rolls, on which it forms solidified xe2x80x9cshellsxe2x80x9d, arrangements being made for the shells to join together in the xe2x80x9cnipxe2x80x9d, that is to say the region where the distance between the rolls is a minimum.
One of the main problems encountered when manufacturing thin stainless steel strip by twin-roll casting is that there is a high risk of surface defects called microcracks appearing on the strip. These cracks are small, but they are nevertheless sufficient to make the resulting cold-converted products unsuitable for use. The microcracks form during solidification of the steel and have a depth of about 40 xcexcm and an opening of approximately 20 xcexcm. Their appearance depends on the contractions of the metal, during solidification of the shells on contact with the rolls over the length of their contact arc. This solidification may be described as having two successive steps. The first step takes place during the initial contact between the liquid steel and the surface of the roll, which results in the formation of a solid steel shell at the surface of the rolls. The second step relates to the growth of this shell as far as the nip, where, as mentioned, it joins the shell formed on the other roll in order to constitute the fully solidified strip. The contact between the steel and the surface of the roll is determined by the topography of the surface of the casting rolls, together with the nature of the inert gas surrounding the casting space and the chemical composition of the steel. All these parameters are involved in establishing the heat transfer between the steel and the roll and govern the conditions under which the shells solidify. As the shells solidify and cool, they undergo contractions. These depend especially on the extent of the xcex4xe2x86x92xcex3 phase transformation, which takes place with a substantial change in the density of the metal, at the microscopic level. It is determined by the composition of the cast metal. These contractions will also modify the shell solidification and cooling conditions.
The Creq/Nieq ratio is conventionally considered as being representative of the solidification path of austenitic stainless steels. It is calculated, using the Hammar and Swensson relationship, by means of the formulae (the percentages are weight percentages):
Creq(%)=Cr%+1.37Mo%+1.5Si%+2Nb%+3Ti%
Nieq(%)=Ni%+0.31Mn%+22C%+14.2N%+Cu%.
Various attempts have been made to develop twin-roll casting processes for obtaining, reliably, strip free of unacceptable surface defects such as microcracks.
With regard to austenitic stainless steels, mention may be made of the document EP-A-0 409 645. This combines a defined geometry of xe2x80x9cdimplesxe2x80x9d (etched valleys of roughly circular or elliptical shape) present on the surface of the rolls with the use as inert gas of a gas mixture containing 30 to 90% of a gas soluble in the steel, which coats the dimples at the moment of the first roll/liquid steel contact. The document EP-A-0 481 481 combines a chemical composition, in which the xcex4-Feca1 index defined by xcex4-Feca1=3 (Cr %+1.5Si %+Mo %)xe2x88x922.8 (Ni %+0.5Mn %+0.5Cu %)xe2x88x9284 (C %+N %)xe2x88x9219.8 is between 5 and 9%, with a dimple geometry on the rolls, so as to encourage solidification as primary ferrite xcex4xe2x86x92xcex4+xcex3. The dimples may conventionally be produced by shot blasting or laser machining. In both the above documents, there is a requirement for these dimples to be separated from one another.
The document EP-A-0 679 114 proposes the use of circumferential grooves made on the surface of the rolls, which give said surface a roughness Ra of 2.5 to 15 xcexcm. It is combined with a chemical composition of the steel allowing solidification as primary austenite, characterized by a Creq/Nieq ratio of less than 1.60. However, solidification as primary austenite increases the hot cracking sensitivity of stainless steels and the risks of forming longitudinal cracks in the strip.
The document EP-A-0 796 685 teaches the casting of a steel whose Creq/Nieq ratio is greater than 1.55 so as to minimize the phase changes at high temperature and to carry out this casting by using rolls whose surface includes touching dimples 100-1500 xcexcm in diameter and 20-150 xcexcm in depth and by inerting the region around the meniscus (the intersection between the surface of the liquid steel and the surface of the rolls) with a gas soluble in the steel, or a gas mixture composed predominantly of such a soluble gas. The roughness peaks serve as sites for initiating the solidification, whereas the valleys of the roughness constitute metal contraction joints during solidification, and allow better distribution of the stresses. However, when the CReq/Nieq ratio is greater than 1.70, it is not always possible to avoid the presence of a few microcracks.
It is an object of the invention to provide a process for the casting of thin austenitic stainless steel strip whose surface is free of microcracks and of other major defects, not requiring particularly demanding casting conditions for implementing it and making it possible to cast steels having a more extended CReq/Nieq ratio than in the existing processes.
For this purpose, the subject of the invention is a process for the continuous casting of an austenitic stainless steel strip having a thickness of less than or equal to 10 mm, directly from liquid metal, between two cooled horizontal rolls, characterized in that:
the composition of said steel, in percentages by weight, comprises: C %xe2x89xa60.08; Si %xe2x89xa61; P %xe2x89xa60.04; Mn %xe2x89xa62; Cr % between 17 and 20; Ni % between 8 and 10.5; S % between 0.007 and 0.040; the balance being iron and impurities resulting from the smelting;
the Creq/Nieq ratio is between 1.55 and 1.90 with:
Creq(%)=Cr%+1.37Mo%+1.5Si%+2Nb%+3Ti%
and
Nieq(%)=Ni%+0.31Mn%+22C%+14.2N%+Cu%;
the surface of the rolls has touching dimples of approximately circular or elliptical cross section, having a diameter of 100 to 1500 xcexcm and a depth of 20 to 150 xcexcm;
the inert gas surrounding the meniscus is a gas soluble in the steel or a mixture of such gases, or consists of at least 50% by volume of such a gas or gas mixture.
The subject of the invention is also strip that can be produced by this process.
As will have been understood, the invention consists in combining conditions relating to the composition of the cast metal, the surface finish of the rolls and the composition of the gas for inerting the meniscus, so as to obtain a strip surface free of microcracks. The main novelty of the composition required is that the metal must contain an amount of sulfur greater than the amounts more usually encountered (without, however, being high to the point of compromising the corrosion resistance of the products) and that this content must be combined with a precise range of Creq/Nieq ratios.